1. Field of the Invention
The present invention relates to a concentric optical system which is usable as either an ocular optical system or an imaging optical system. More particularly, the present invention relates to a concentric optical system which provides a high resolution, a wide field angle and a large pupil diameter with minimal chromatic aberration.
2. Background of Related Art
A Schmidt system, which is well known as an objective for astronomical telescopes, is generally arranged such that an aspherical lens which is close to a plane-parallel plate is placed at the spherical center of a concave mirror to correct spherical aberration, and a diaphragm is disposed at the spherical center to correct coma and astigmatism.
Concentric optical systems, which are represented by the above Schmidt system, are free from coma and astigmatism by virtue of the diaphragm disposed in the vicinity of the center of curvature of the concave mirror. However, since the curvature of field cannot be corrected, a large field curvature occurs.
U.S. Reissued Pat. No. 27,356 discloses an ocular optical system which uses a semitransparent concave mirror and a semitransparent plane mirror to project an object surface at a distance, and which adopts an arrangement wherein the field curvature produced by the semitransparent concave mirror is corrected by curving the object surface. In U.S. Reissued Pat. No. 27,356, however, the semitransparent concave mirror and the semitransparent plane mirror are each formed from a single independent constituent element. Therefore, when the field angle is widened, off-axis aberrations such as astigmatism, coma, etc. are likely to occur. In addition, the radius of curvature of the semitransparent concave mirror reduces, which is unfavorable from the manufacturing point of view in actual practice.
There has also been a known arrangement in which light rays are reflected by a forward mirror to once turn back the optical axis, and the reflected rays are reflected again by another mirror. The arrangement is known as a reflecting telephoto objective. FIG. 24 is a sectional view showing U.S. Pat. No. 3,700,310 as one example of the reflecting telephoto objective. Referring to the figure, rays successively pass through lenses A, B, C and D and are reflected by a mirror 12. The reflected rays pass through the lenses D and C and are reflected by a mirror 13. Then, the reflected rays successively pass through the lenses C, F, G, H and I to form an image.
In the conventional reflecting telephoto objective, however, the mirrors 12 and 13 are totally reflecting mirrors. Therefore, it is necessary in order to prevent a bundle of rays entering through the foremost surface R.sub.1 from being cut by the mirror 12 or 13 to increase the size of the mirrors 12 and 13 or to provide the pupil position in the range of from the foremost surface R.sub.1 to the rearmost surface R.sub.6. Accordingly, the pupil of the conventional reflecting telephoto objective is provided between the foremost and rearmost surfaces of the entire lens system to prevent the image from darkening without an increase in size of the entire lens system. In the above-described U.S. Pat. No. 3,700,310, the pupil is provided on the surface R.sub.6.
However, since it is necessary to take the turn-back optical paths into consideration when installing the mirrors, the degree of freedom is disadvantageously low, and the aberration correcting capability is also deteriorated. In addition, different lenses are used for each turn-back optical path, such as the lenses A and B and the central lens E, and the lens D and the lenses F to I, which are disposed in the center thereof, in U.S. Pat. No. 3,700,310. Accordingly, the arrangement is complicated, and thus the conventional optical system involves problems such as reduction in productivity, rise in cost, etc.